This invention is related to two commonly assigned applications by one of the coinventors herein, entitled PIGMENT ENCAPSULATED LATEX AQUEOUS COLORANT DISPERSIONS, Ser. No. 06/842,609, now U.S. Pat. No. 4,665,107, and ELECTROPHORETIC DISPLAY PARTICLES AND A PROCESS FOR THEIR PREPARATION, Ser. No. 06/842,608, which were filed Mar. 21, 1986.
1. Field of the Invention
The present invention involves specific improvements to the known concept of combining a dye, which is a molecular coloring agent, with the molecules of a particular form of polymer, for the purpose of making submicron sized particles of polymer (hereafter "nanoparticles" or "nanospheres") with the dye molecules in a homogeneous mix.
2. Brief Description of the Prior Art
The present invention relates to dye molecules, which are not encapsulated as solid cores within a polymer shell. The present invention, therefore, is in categorical distinction to the pigment encapsulated latex teachings presented in the two above-identified applications. The embodiments of the present invention improve upon the known concept of molecular mixing of a dye homogeneously throughout a polymer, through the specific improvement of an epoxy based thermoset polymer system, whereupon any crosslinking can be made selectively to occur upon the selective addition of heat, or catalyst, after nanospheres of dyed polymer have been created by a solvent removal step.
The categories of dyes useful with the present invention are those that are water insoluble, and soluble within certain solvents for the polymer. At least the dye and epoxy resin first are dissolved together in a mutual solvent in order to create a solution of dye and resin, which then is dispersed as a discontinuous phase of nanodrops in an aqueous continuous phase. The degree of emulsification is assisted by the use of surfactants and other forms of dispersing agents.
There are conventional, water-based, low-viscosity writing fluids which comprise a dye dissolved in the water, which may or may not contain a dissolved water-soluble polymer to provide film-forming characteristics to a line that is to be drawn upon a substrate. It further is known to create writing fluids wherein a water insoluble dye is mixed with film-forming polymers, (such as polyvinyl, vinyl acetyl polymers, styrene-maleic anhydride copolymers, and alkylated vinylpyrrolidone copolymers,) in an organic solvent; the solution of dye and polymer are then dispersed in water to obtain an emulsion; and then the solvent removed to create a dyed polymer dispersion in water. Such prior formulations have been found to be suitable when used in certain writing fluid applications, though other writing fluid--and additional applications--require polymer characteristics beyond the limits of thermo-plastic resins.
OKA, U.S. Pat. No. 4,588,617, illustrates a technique where a bisphenol-A type glycidyl ether/amine copolymer system is used to produce cured epoxy resin spherical particles of less than 50 microns, by adding a water-soluble amine curing agent to an emulsion of uncured epoxy resin. One after-treatment is to dye the cured particles with an acid dye.
The present invention uses a solvent system to initially intermix dye and polymer molecules. The dyed particles also are not limited in size to that dictated by the size of the solid core material, as in U.S. Pat. No. 4,665,107. Further, the dye molecules are homogeneously permeating within a cross-linked polymer system and are available for use in a significantly stabilized form, that significantly resists penetration into and adhesion onto a substrate to which it may be applied. Alternatively, the uncured nanoparticles of the invention can be maintained in an aqueous solution and allowed to become crosslinked upon application, by external heat or the addition of a curing agent. After curing, the applied nanoparticles will maintain the stability typical of the thermoset polymer system.
The present invention also categorically is not an emulsion polymerization, since there are no primary particles which in turn are dispersed in a monomer (or mixture of monomers) in order to provide sites for a polymerization of polymer around those particles. An example of a prior art technique for producing colloidal sized, hydrophobic polymer particles which surround discrete particles of an inorganic material--through such an emulsion polymerization--is represented by SOLC nee Hajna, U.S. Pat. No. 4,421,660.
The use of an epoxy resin thermoset polymer system to polymer-coat solid core particles is disclosed in U.S. Pat. No. 4,665,107, so that the coated particles may be rendered stable after being applied to a substrate. However, the present improvements focus upon a polymer cross-linking system that also may create stable dyed polymer particles at any time, even in aqueous solution. Further, intermixed dye polymer particles will show a reduced tendency to settle from the continuous water phase, when compared to solid core materials, due to reduced density differentials of dyes to the continuous water phase. With the present invention, a selective delay to the step of cross-linking is enabled, even through predictable fluid rheology and surface morphology can still be achieved for optimum pre and post cure nanoparticle behavior. The product of the process ultimately is a cured, dyed polymer, but the circumstances of curing can be delayed even until after a film has been applied to a substrate.